As I mentioned previously in this forum; I have written a Vizard plugin for FaceAPI (a very accurate markerless head-tracking API).
I have several commercial production licenses (with dongle which unlockes all options/features) which I offer up for sale bundled with the plugin. This license cannot expire.

You can track a head with it using a standard webcam, detect nodding, point to objects by moving your head, detect movement/position of the eyes/eyebrows/lips and other facial features and much more. All very accurate with high updaterates.
Also a very precise fishtank is possible. Look here (click on VirtualWindow).